The present invention relates to an apparatus for producing interferometric fringe patterns. In particular, the present invention is directed to an apparatus for producing interferometric fringe patterns such that the contrast, spatial frequency, temporal frequency, intensity, and orientation of the interferometric fringe pattern can be selectively varied.
The production of interferometric fringe patterns is relatively well-known. For example, interferometric fringe patterns produced by shearing interferometers and other known devices are used in a variety of applications, including pattern electroretinogram testing, as disclosed and claimed in U.S. Pat. No. 5,233,373 to Peters, et al. issued Aug. 3, 1993. Interferometric fringe patterns can also be used in conjunction with visual acuity and contrast sensitivity testing, as disclosed in U.S. Pat. No. 5,216,458 to Andera, et al. issued Jun. 1, 1993.
Known devices and methods for producing fringe patterns typically employ polychromatic illumination of diffraction gratings, Moire fringes, or shearing interferometry to produce the desired fringes. However, such known devices and methods have inherent limitations in their application due to the limited number of parameters which can be varied as well as the limited range and resolution over which these parameters can be varied.
For example, the device disclosed in U.S. Pat. No. 5,216,458 to Andera, et al. generates a variable contrast and spatial frequency fringe pattern to the eye for the purpose of measuring visual acuity and contrast sensitivity. However, the number of spatial frequencies that can be tested using a device of this type is limited by the number of spatial frequency "optotypes". In addition, the device provides no mechanism for the variation of the temporal frequency or the orientation of the fringe pattern. Furthermore, the device disclosed by Andera, et al. does not use a monochromatic light source and is thus susceptible to test errors caused by ocular chromatic aberrations.
Known devices and methods for producing fringe patterns are typically inefficient with respect to light transmission. For example, a shearing interferometer typically loses approximately 90%-95% of the intensity of the source light when used to create interferometric fringe patterns. Due to this relatively inefficient use of the source light, a relatively high-powered light source is required in order to generate a viable fringe pattern. The need for a higher powered light source increases the power requirements of the apparatus as well as increasing the space occupied by the device.